Recently, with miniaturization of electronic devices, demands for a high-capacity secondary battery are increasing. In particular, a lithium ion secondary battery having a high energy density and excellent large-current charge/discharge characteristics as compared with a nickel-cadmium battery or a nickel-hydrogen battery is attracting attention.
As for the negative electrode material of a lithium ion secondary battery, a graphite material or an amorphous carbon is often used in view of cost and durability. However, the reversible capacity of the amorphous carbon material is low as long as practically applicable materials are concerned, or the graphite material has a problem that when the active material layer containing a negative electrode material is highly densified so as to obtain high capacity, the charge/discharge irreversible capacity in the initial cycle increases due to material fracture, failing in obtaining high capacity.
In order to solve such a problem, for example, a technique of heat-treating the graphite material is known. Patent Document 1 discloses heat-treating scale graphite at 400 to 1,800° C. in an argon atmosphere. Patent Document 2 discloses heat-treating graphite at a temperature over 2,400° C. Patent Document 3 discloses heat-treating scale graphite at 900° C. or less to have a rhombohedral structure fraction of 20% or less in the graphite. Patent Document 4 discloses treating natural graphite at 2,000° C. in an inert atmosphere. The techniques disclosed in these patent documents, where scale graphite is used as the raw material, are expected to achieve reduction in the irreversible capacity attributable to a negative electrode material, which would be obtained by heat treatment, but requirement for rapid charge/discharge characteristics, arising with increase in the negative electrode density, is not sufficiently responded to.
Also, Patent Document 5 discloses a negative electrode material caused to have a ringed structure composed of two or more layers on an end face by a heat treatment. In this negative electrode material, the reactivity with an electrolytic solution is suppressed, but the end face allowing for entrance and exit of Li is also closed by a heat treatment at ≧2,500° C. and therefore, the rapid charge/discharge characteristics are still insufficient.
Patent Document 6 discloses a graphite particle in which the amount of an acidic functional group is 5 milliequivalent/kg or less and 0.3 μmol/m2 or less. Patent Document 7 discloses graphite in which the Tap density is from 0.8 to 1.35 g/cm3, the abundance ratio O/C of oxygen to carbon determined by XPS is less than 0.01, the specific surface area SA determined using N2 is from 2.5 to 7 m2/g or less, and the Raman R value is from 0.02 to 0.05. In Patent Documents 6 and 7, a spheroidized graphite particle is used, but the amount of an acidic functional group is too small and rapid charge/discharge characteristics surpassing the spheroidized graphite as the raw material can be hardly obtained, though an effect of reducing irreversible capacity may be expected similarly to the above-described techniques of heat-treating scale graphite.
Furthermore, Patent Document 8 discloses a particle obtained by rapidly heating and rapidly cooling a spheroidized graphite particle in a non-oxidizing atmosphere. However, as described later, the crystal structure on the end face of graphite is insufficiently changed by the rapid heat treatment and the effect of responding to rapid charge/discharge characteristics is lacking.
In this way, there is conventionally a trade-off relationship in the graphite-based material, that is, when the end face of a graphite particle is inactivated so as to suppress an excessive reaction between an electrolytic solution and the particle, the rapid charge acceptance characteristics tend to be deteriorated, whereas for enhancing the rapid charge acceptance characteristics, the specific surface area must be increased and this involves an excess consumption of electrolytic solution and gives rise to electrolyte depletion or the like. The carbon material of the present invention can satisfy both of these characteristic features that have been heretofore considered contradictory, and can achieve both rapid charge/discharge characteristics and reduction in the irreversible capacity.
Also, in general, a negative electrode using an amorphous carbon material as the active material is excellent in the rapid charge/discharge characteristics of Li. However, there is a problem that since the potential curve during charging passes through a noble potential compared with the potential curve of a graphite material, a potential difference with the positive electrode cannot be created when a battery is fabricated. Furthermore, the amorphous carbon material lacks in slipperiness at the electrode rolling and therefore, is improper when producing a high-density negative electrode with an aim to obtain high capacity.